Dental crown and bridge restorations are often made with a metal base having a jacket or covering of dental porcelain so that the restoration will closely resemble a natural tooth. Such restorations are well known and have been used for many years.
The general technique for the construction of a porcelain coated dental restoration (i.e. crown or bridge), involves first taking an impression of a denture area that has been prepared to receive the restoration. A die is prepared from the impression, and a metal base ("coping") is cast to fit this die. The metal base has an internal shape to match the prepared denture. A porcelain powder is then mixed with water to form a slurry which is then applied to the metal base by standard procedures. The slurry is shaped in the form of the finished crown or multiple unit bridge. The porcelain is then dried, and fired in a furnace at a desired firing temperature. The crown or bridge may be fired several times before the final form is obtained, and the porcelain may be applied in several layers.
There is a significant temperature change from the firing temperature to room temperature, as a restoration is alternately cooled and fired. Therefore, significant stress can be induced in the restoration if the thermal expansion of the porcelain coating does not closely match that of the metal base.
Metal bases that are most often employed today in such restorations include gold, high and low gold alloys including gold-palladium alloys, silver-palladium alloys, high palladium alloys, nickel-chrome-molybdenum type alloys, gold-silver-palladium alloys and palladium-copper alloys. Gold and its alloys are preferred metals for a metal base due to their biocompatibility with the human body. Gold, however, is a very expensive metal and, like other metals typically used for dental restorations, it requires high fusing temperatures to bond with a jacket or covering of dental porcelain. These metals and alloys exhibit thermal expansion coefficients of about 14.times.10.sup.-6 in/in/.degree. C. and thus ceramics used in combination therewith have similarly high thermal expansion coefficients.
Current commercially available dental porcelains have fusion temperatures in the range of 1700.degree. F. and up. High fusion temperatures of today's dental porcelains prevent their use with certain metals which readily oxidize at high temperatures.
It is therefore desirable to provide a dental restoration which comprises a metal base, or coping, which is biocompatible with the human body and which is not very expensive. It is also desirable to provide a dental porcelain which may be used with an inexpensive, biocompatible metal having a low fusion temperature. The present invention also relates to a dental porcelain which has a low fusing temperature and which bonds securely to a metal base.